Cold flow test conducted at the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) was numerically simulated to investigate flow transition between two operating modes of a dual-bell nozzle. Through unsteady Reynolds Averaged Navier-Stokes simulation (RANS) based on the realizable k-ε turbulence model, flow transition from the base nozzle to the extension nozzle was observed, but the nozzle pressure ratio (NPR) at the transition was higher than the experiment. Quasi-steady RANS simulations were conducted to clarify the effect of the RANS turbulence model. It was found that the recirculation flow generated behind the separation was affected by the feature of the RANS turbulence model, which resulted in the difference of the transition NPR.会議情報: 第59回宇宙科学技術連合講演会（2015年10月7日-9日. かごしま県民交流センター), 鹿児島市, 鹿児島県形態: カラー図版ありPhysical characteristics: Original contains color illustrations